High frequency resonant inverters are typically used in electronic ballasts to power gas discharge lamps. One advantage of voltage fed resonant inverters with MOSFETs is high efficiency associated with Zero Voltage Switching (ZVS) and with low drain to source resistance of power MOSFETs when conducting. These inverters provide almost sinusoidal current to the gas discharge lamps that is very important for longer lamp life.
A typical resonant inverter comprises a half bridge switching circuit with power MOSFETs generating high frequency AC to power a resonant load with at least one gas discharge lamp. Ballast voltage fed resonant inverter utilizes a series LC resonant tank circuit with the load connected in parallel to a resonant capacitor. A control circuit provides MOSFET switching frequency above a resonant frequency for zero voltage turn on. When switching above resonance, the input of the resonant load is inductive. When switching below resonant, this input is capacitive and should be avoided because it is associated with hard switching.
In the previous Osram Sylvania U.S. Pat. Nos. 6,090,473, 7,030,570 and 7,045,966, several ballast inverter control circuits have been proposed that employ standard industrial controllers and self-oscillating half bridges, for instance, the IR215X series and the IR53H(D) series from International Rectifier, UBA2024P from Philips, L6579 series from ST Microelectronics, etc.
Accordingly to above, self oscillating drivers-controllers utilize phase locked feedbacks for guaranteed soft switching of inverter transistors. Another advantage of the feedbacks is a possibility for dimming by a small signal DC bias. Also, Osram Sylvania U.S. Pat. No. 6,545,432 discloses lamp-out sensor with a series to lamp capacitor to shut down the resonant inverter after occurrence of a lamp-out condition.
In the above mentioned Osram Sylvania patents, ballast inverter circuits are illustrated that power a single discharge lamp. However, there is a big demand for an instant start multi-lamp ballast inverter powering several lamps simultaneously. In some applications the ballast should meet lamp Hot Swap requirements and continue to operate without interruption when a lamp is removed. One of solutions to the problem is described in the publication “High-Efficiency Low-Stress Electronic Dimming Ballast for Multiple Fluorescent Lamps”, Tsai-Wu et all, IEEE Transactions on Power Electronics, Vol. 14, NO. 1, 1999. This ballast utilizes multiple resonant loads connected in parallel in the resonant inverter.
Having multiple resonant inductors and capacitors may not be a cost effective solution. U.S. Pat. No. 6,362,575 issued to Chang et al. discloses a single resonant inductor and a single resonant capacitor ballast inverter for multiple discharge lamps. The voltage fed ballast inverter provides high frequency regulated voltage for discharge lamps each connected in series with a ballasting capacitor. This inverter has a single mode operation capability. Because output regulated voltage should be always high (at least, 600V AC for T8 lamp) to provide starting, the ballast has increased power losses in steady-state operation, basically in the resonant inductor.
Therefore, to reduce inverter output voltage with several parallel lamps, there is a need for a more efficient stepped control in programmed controller operations. A control circuit with wider frequency range for the inverter to operate the resonant load with low Q, especially for inverter operating from high voltage DC Bus (400V and higher), would be preferable in certain configurations.